Ink jet printing has in recent years become a very important means for recording data and images onto a paper sheet. Low costs, easy production of multicolor images and relatively high speed are some of the advantages of this technology. Ink jet printing does however place great demands on the substrate in order to meet the requirements of short drying time, high print density and sharpness, and reduced color-to-color bleed. Furthermore, the substrate should have a high brightness. Plain papers for example are poor at absorbing the water-based anionic dyes or pigments used in ink jet printing; the ink remains for a considerable time on the surface of the paper which allows diffusion of the ink to take place and leads to low print sharpness. One method of achieving a short drying time while providing high print density and sharpness is to use special silica-coated papers. Such papers however are expensive to produce.
U.S. Pat. No. 6,207,258 provides a partial solution to this problem by disclosing that pigmented ink jet print quality can be improved by treating the substrate surface with an aqueous sizing medium containing a divalent metal salt. Calcium chloride and magnesium chloride are preferred divalent metal salts. The sizing medium may also contain other conventional paper additives used in treating uncoated paper. Included in conventional paper additives are optical brightening agents (OBAs) which are well known to improve considerably the whiteness of paper and thereby the contrast between the ink jet print and the background. U.S. Pat. No. 6,207,258 offers no examples of the use of optical brightening agents with the invention.
WO 2007/044228 claims compositions including an alkenyl succinic anhydride sizing agent and/or an alkyl ketene dimmer sizing agent, and incorporating a metallic salt. No reference is made to the use of optical brightening agents with the invention.
WO 2008/048265 claims a recording sheet for printing comprising a substrate formed from ligno cellulosic fibers of which at least one surface is treated with a water soluble divalent metal salt. The recording sheet exhibits an enhanced image drying time. Optical brighteners are included in a list of optional components of a preferred surface treatment comprising calcium chloride and one or more starches. No examples are provided of the use of optical brighteners with the invention.
WO 2007/053681 describes a sizing composition that, when applied to an ink jet substrate, improves print density, color-to-color bleed, print sharpness and/or image dry time. The sizing composition comprises at least one pigment, preferably either precipitated or ground calcium carbonate, at least one binder, one example of which is a multicomponent system including starch and polyvinyl alcohol, at least one nitrogen containing organic species, preferably a polymer or copolymer of diallyldimethyl ammonium chloride (DADMAC), and at least one inorganic salt. The sizing composition may also contain at least one optical brightening agent.
The advantages of using a divalent metal salt, such as calcium chloride, in substrates intended for pigmented ink jet printing can only be fully realized when a compatible water-soluble optical brightener becomes available. It is well-known however that water-soluble optical brighteners are prone to precipitation in high calcium concentrations. (See, for example, page 50 in Tracing Technique in Geohydrology by Werner Käss and Horst Behrens, published by Taylor & Francis, 1998).
Accordingly, there is a need for improved optical brightening compositions which have good compatibility with sizing compositions containing a divalent metal salt.